Numerical investigations of fluid flow and heat transfer processes in the internal structures of thermoacoustic devices
thesisposted on 23.12.2013, 10:48 by Fatimah Al Zahrah Mohd Saat
Thermoacoustic devices are built based on interactions between sound wave and a solid boundary, within a well-controlled environment, to produce either power or cooling effect. Recent studies on two important internal structures, namely regenerator and heat exchangers, are reviewed. Furthermore, the need for detailed investigations on a pressure drop condition in the flow through the regenerator and heat transfer condition in heat exchangers working in a thermoacoustic environment is also addressed. A two-dimensional porous medium model is developed based on the pressure drop measurement of a regenerator working in a well-controlled travelling-wave time-phasing, wherein the pressure and velocity of the oscillatory flow across the regenerator are controlled to be in-phase. A friction correlation is proposed based on Darcy’s law. The model is developed in a commercial software ANSYS FLUENT to determine a permeability coefficient for the model. The findings suggest that a steady-state correlation is applicable provided that the travelling-wave time-phasing is met. Otherwise, a phase-shift effect should be considered and the steady-state approximation may no longer hold true. A pair of adjacent plate heat exchangers in the oscillatory flow is studied. It is shown that the application of the temperature difference between “cold” and “hot” plates leads to interesting asymmetries within the flow field. Also a need for a turbulence model at a drive ratio lower than suggested in current literature is discovered and discussed. It is found that the heat absorbed by the cold plate is lower than the heat supplied by the hot plate and heat accumulation is observed in the system. The vortex structures and viscous dissipation change with operating conditions. The combined effect of flow amplitude, natural convection and the “annular effect” of velocity profiles near the channel wall on the flow are discussed. A good agreement with experimental results obtained previously is shown.